User Manual
Chapter 7
GFK-1503F
Jan 2020
CPU Operation
73
System
Communications
Window
Next, the CPU processes communications requests from intelligent option
modules. The modules are polled in round-robin fashion, so no module has
priority.
In default (“Run to Completion”) mode, the length of the system communications
window is limited to 400 milliseconds. If a module makes a request that requires
more than 400 milliseconds to process, the request is spread out over multiple
sweeps.
In Limited mode, option modules that communicate with the PLC using the
system window have less impact on sweep time, but response to their requests is
slower.
Diagnostics
A checksum calculation is performed on the application program at the end of
every sweep. You can specify from 0 to 32 words to be checksummed. If the
calculated checksum does not match the reference checksum, the program
checksum failure exception flag is raised. A fault is entered in the PLC fault table
and the PLC goes to Stop mode. If the checksum calculation fails, the
programmer communications window is not affected.
Each sweep, the CPU verifies the physical configuration of one module against its
programmed configuration. A missing, additional, or mismatched module
causes a fault to be generated.
7.2
Standard CPU Sweep Operation
Standard Sweep operation is the normal operating mode of the PLC CPU. In Standard
Sweep operation, the CPU repeatedly executes the application program, updates I/O, and
performs communications and other tasks shown in the diagram:
1.
The CPU performs its start-of-sweep housekeeping tasks.
2.
It reads inputs.
3.
It executes the application program.
4.
It updates outputs
5.
If a programming device is present, the CPU communicates with it.
6.
It communicates with other devices.
7.
It performs diagnostics
Except for communicating with a programmer, all these steps execute every sweep.
Programmer communications occur only when needed.
In this mode, the CPU performs all parts of its sweep normally. Each sweep executes as
quickly as possible with a different amount of time consumed each sweep.
Содержание PaCSystems VersaMax Series
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Страница 85: ...User Manual Chapter 7 GFK 1503F Jan 2020 CPU Operation 71 7 1 Parts of the CPU Sweep Figure 47 ...
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